package com.lyz.dataStructure.algorithm.yitu2020;/**
 * @Author:759057893@qq.com Lyz
 * @Date: ${Date} 19:53
 * @Description:
 **/

import java.util.Scanner;

/**
 *@Author:759057893@qq.com Lyz
 *@Date: 2019/8/9 19:53
 *@Description:
 **/
public class Main2 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n =sc.nextInt();
        System.out.println(xiangzhuang(n));

    }
    public static  int xiangzhuang(int n){
        if(n<4) return 0;
        if(n==4) return 2;
        double a = 1.5;
        double b =1;
        double c =0;
        for(int i =3;i<n;i++){
            c =a+b;
            b = a ;
            a =c;
        }

        return  (int)c;
    }
}
